In today's business and scientific world, color has become essential as a component of communication. Color facilitates the sharing of knowledge and ideas. Companies involved in the development of digital color printing engines are continuously looking for ways to improve the total image quality of their products. One element that affects image quality is the ability to consistently produce the same quality image output on a printer from one day to another. Users have become accustomed to printers and copiers that produce high quality color and gray-scaled output. Users expect to be able to reproduce a color image with consistent quality on any compatible marking device. There is a commercial need for efficiently maintaining print color predictability and image reproduction quality, particularly as electronic marketing has placed more importance on the accurate representation of merchandise in illustrative print or display media.
In conventional marking devices, quality defects appear in a marked image due to various factors. For example, changes in the optical density introduced by differences in laser/LED bar intensities, subsystem non-uniformities, donor roll reload, and the like can lead to image quality defects. Additionally, spatial non-uniformity errors, e.g., a situation in which pixels in one part of an image that have been defined as a certain color appear different from pixels in another part of the image that have been defined as the same color, are also present. Wire history, wire contamination, charging subsystem variations and photoreceptor variations are among the root causes for spatial non-uniformity errors in images produced by xerographic printers.
Spatial non-uniformity errors can be addressed by modifying hardware or hardware operations. For example, in LED bars, exposure variations can be minimized by measuring the output of the LED elements and adjusting their duty cycle and/or intensity to ensure that all the elements have the same output. In laser exposure systems, similar duty cycle adjustments can be performed to minimize the exposure-related non-uniformities. Furthermore, routine cleaning of wires to remove contamination helps to reduce wire history-related non-uniformities.
Direct marking printers, such as ink jet printers, can produce images with banding due to a variation in drop mass from nozzle to nozzle in the printhead of the marking engine. One aspect of a direct marking process is that the degree of banding depends on the screen used in writing a particular gray level. For example, the banding arising from a 1 on 1 off ladder chart will be different from the banding resulting from a 50% error diffused screen. One cause of this difference is drop coalescence, i.e., drops jetting one after another or adjacent to each other in different pixel columns may coalescence. The tendency to coalesce depends on the drop's mass. One screen may produce many regions where large drops coalesce while another screen may not, so these two screens can have a different degree of banding.
When primary colors are combined to form secondary colors or process black, intensity or hue banding may still appear due to a change in dot pattern used for any given primary color when it is part of a secondary color. This pattern change may be intentional and can result from a desire to avoid placing primary color dots on top of each other when forming secondary colors.
Accordingly, what is needed in this art are increasingly sophisticated systems and methods which reduce process color banding caused by printhead non-uniformities in direct color marking devices.